1. Field of the Invention
The present invention relates generally to proton electrolyte membrane (PEM) fuel cells and specifically to a method and apparatus for cooling and humidifying fuel cells using anti-freeze coolant.
2. Discussion of the Prior Art
In an effort to find new energy sources, fuel cells using an electrochemical reaction to generate electricity are becoming an attractive energy alternative. Fuel cells offer low emissions, high fuel energy conversion efficiencies, and low noise and vibrations. U.S. Pat. No. 5,248,566 to Kumar et al. These advantages make fuel cells useful in automotive applications. Of the various types of fuel cell types, the proton electrolyte membrane (PEM) fuel cell appears to be the most suitable for use in automobiles, as it can produce potentially high energy, but has low weight and volume.
Solid PEM fuel cells are well known in the prior art and generally employ a membrane electrode assembly (MEA) that consists of electrodes contained in an anode compartment and a cathode compartment separated by a solid polymer electrolyte membrane. The MEA for each fuel cell is sandwiched between a pair of electrically conductive elements. These elements serve as current collectors for the electrodes (anode and cathode). They contain an array of grooves in their faces for distributing the fuel cells"" gaseous reactants over the surfaces of the respective anode and cathode. The MEA contains a layer of catalyst at each membrane/electrode interface to induce the desired electrochemical reaction.
In operation, the electrodes are electrically coupled. When coupled, the electrodes provide a circuit for conducting electrons between the electrodes through an external circuit.
Two or more fuel cells can be electrically connected together in series to increase the overall power output of the assembly. Such a multiple fuel cell arrangement is referred to as a fuel cell stack. The stack typically includes inlet ports and manifolds for directing a fluid fuel stream and a fluid oxidant stream to the individual fuel cell reactant flow passages.
The fuel cell stack also commonly includes an inlet port and manifold for directing a coolant fluid stream, typically water, to interior passages within the stack to absorb heat generated by the fuel cell during operation.
The stack also generally includes exhaust manifolds and outlet ports. The ports are needed to expel the depleted reactant streams, reaction products such as water, and an exhaust manifold and outlet port for the coolant stream exiting the stack.
Each reactant stream exiting the fuel cell stack generally contains water. The outlet fuel stream from the anodes generally also contains water added to humidify the stream plus any product water drawn across the membrane from the cathode. The outlet oxidant stream from the cathodes generally contains water added to humidify the stream plus product water formed at the cathode.
In fuel cells it would initially appear that since water is the product of the reaction, sufficient water would be already present. Unfortunately, the water formed in a fuel cell is inadequate to maintain membrane hydration except under special conditions. Each proton that moves through the membrane drags at least two or three water molecules with it. As the current density increases, the number of water molecules moved through the membrane also increases. Eventually, the flux of water being pulled through the membrane by the proton flux exceeds the rate at which water is replenished by diffusion. At this point, the membrane begins to dry out, and its internal resistance increases. This sets a relatively low limit on the current density that can be maintained by back diffusion from the cathode surface.
The need to add water (humidify) to support proton conduction has been addressed in a number of ways. See generally, U.S. Pat. No. 5,996,976 to Murphy et al. Use of a solid polymer (or composite polymer-inorganic material) membrane has been developed to humidify the fuel cell accordingly.
The presence of water in the fuel cell system, especially in an automotive application in a cold climate, creates design challenges. The water in the fuel cell used for cooling and humidification can freeze when not in use. At a minimum, the ice would need to be melted prior to operation to resolve this problem. This requires energy, impeding the goal of reduced fuel consumption. Further, the expansion of the water during freezing could expand and damage the fuel cell system.
Various methods and devices have been proposed in the prior art for addressing the freezing problem. U.S. Pat. No. 5,798,186 to Fletcher et al. discloses a method for starting the fuel below the freezing temperature of water and allowing the exothermic fuel cell reaction to raise the stack core temperature above the freezing temperature of water.
U.S. Pat. Nos. 5,965,288 and 6,040,073 to Okamoto disclose a solution to the fuel cell freezing problem using independent passageways. This requires the use of water as well as a second medium. The inventions do not address the related problem of keeping the water from freezing.
U.S. Pat. No. 6,068,941 to Fuller et al. describes a method for protecting a PEM fuel cell in a freezing environment by introducing a controlled amount of a low molecular weight alcohol, such as methanol or ethanol, into the coolant passages upon shutdown of the fuel cell in order to prevent water trapped therein from freezing. Upon start-up, air is fed into the cathode reactant flow field and the alcohol diffuses to the cathode catalyst where it is oxidized, thus heating the fuel cell to a normal operating temperature. This method will prevent water trapped in a fuel cell from freezing but will undesirably increase system complexity.
All known prior art retain, to some degree, a pure water channel. Accommodating freezing conditions are thus complex. There remains a need for an improved fuel cell coolant and humidifying system that can accommodate freezing temperatures during periods of non-use without increase in system complexity or loss of system efficiency.
Accordingly, an object of the present invention is to provide an improved PEM fuel cell power plant that eliminates the need for pure water channels.
Specifically, the present invention is a proton electrolyte membrane (PEM) fuel cell system having a closed coolant path within the fuel cell; a humidifier comprising a humidification fluid flow path and a fuel and air gas supply passage continuous in, through and out of the humidifier; the humidification fluid flow path and the fuel and air gas supply passage separated by a water permeable membrane that is impervious to organic materials allowing water from the humidification fluid flow path to enter the fuel and air gas supply passage; and the humidifier connected to the fuel cell by a humidified air and fuel passageway. The water permeable membrane can be a polymer, an inorganic polymer or be made of functional materials such as keggin ions pillared xcex1-ZrP composite materials.
In one embodiment, the coolant path can be configured to include a coolant pump and a heat exchanger. The coolant can be a water and organic material mixture (using, for example glycol) thus allowing coolant flow in temperatures below freezing point.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.